1. Field of the Invention
This invention relates to optical waveguides, and in particular to optical waveguides having an internal grating for phase matching through mode dispersion for a pair of fundamental and second harmonic wavelength modes. The present invention also relates to a method of making such optical waveguides.
2. Related Art
Waveguided nonlinear optics has grown rapidly since the laser was invented. Most postulated phase-matching techniques for frequency mixing have been demonstrated in bulk media and refinements peculiar to waveguides have been exploited in many different configurations. Phase-matching in second-harmonic generation refers to the condition when the fundamental wavelength and the generated second-harmonic wavelength fields propagate in a material in such a direction that both experience the same refractive index. The most recent interest has been in the applications of periodic structures for phase-matching. Although phase-matching to radiation modes of a waveguide can allow the use of the largest second-order nonlinear tensor coefficient in materials, the only known practical technique for phase-matching guided-waves in frequency mixing experiments uses periodic structures. In this respect periodic structures offer the most elegant phase-matching technique for use with waveguides. The confined optical fields with their correspondingly high intensities propagated over long lengths can be exploited optimally.
The scheme for phase-matching based on the use of periodic structures has been known in nonlinear optics for over 20 years. It has been theoretically shown that if the sign of the second-order nonlinear susceptibility was reversed at exactly the coherence length l.sub.c, at which point the `free-wave` and `bound` wave get out of phase, then the effects of dispersion can be compensated, and the material is artificially phase-matched. It is also known that by modulating the refractive index in a spatially periodic fashion a similar result can be achieved, albeit less efficiently. The principle of periodic phase-matching can be understood in the following way. As the fundamental wavelength field propagates through a nonlinear material it generates a second-harmonic wavelength polarisation field which travels at the same velocity as the fundamental wavelength field and is referred to as the `bound` wave (being bound to the fundamental wavelength field). The radiated field at the second-harmonic wavelength is referred to as a `free` wave and travels at a phase velocity determined by the refractive index at the second-harmonic wavelength. The `free` and `bound` waves interfere and periodically exchange energy after an accumulated phase difference of n.pi./2 (where n is an integer). A .pi./2 phase difference is accumulated in a distance of a coherence length. If the phase of the bound wave were changed by .pi./2 every coherence length the `free` wave could be made to grow rather than interferring destructively. The periodic change in the sign of the nonlinear coefficient in the fibre changes the phase of the frequency doubled `bound` wave and therefore gives rise to `periodic` or `quasi phase-matching`.
Recently, an interesting phenomenon was discovered by Hill et al who noted (see Hill K. O., Kawasaki B. S., Johnson D. C. and MacDonald R. I., "CW three wave mixing in single mode optical fibres" J Appl Phys, 49(10), 5098-5106, October 1978) that optical fibres could frequency double light when pumped with high power-densities despite being centro-symmetric. This effect was then investigated by Osterberg and Margulis W. (see: a) Digest of XIV International Quantum Electronics Conference (OSA), Paper WBB2, pp102, Washington DC, 1986; and b) "Dye laser pumped by Nd:YAG laser pulses frequency doubled in glass optical fibre", Optics Letts, 11,516-518, 1986) who showed that not only was the fibre capable of frequency-doubling, but that the frequency-doubled light grew with time to levels of a few percent of the pump power. This observation could only occur if the process was phase-matched. It was later postulated to be achieved through a periodically written second-order susceptibility, X.sup.(2), grating which had a wave-vector equivalent to the momentum mismatch between the pump and the frequency doubled light.
This self-seeded quasi-phase matching (and equivalent externally seeded) scheme in optical fibres is wavelength sensitive and the sensitivity to frequency doubling rapidly tails off at wavelengths longer than 1064 .mu.m for germania doped silica fibres.